The detection and/or quantitation of specific nucleic acid sequences is an increasingly important technique for identifying and classifying microorganisms, diagnosing infectious diseases, detecting and characterizing genetic abnormalities, identifying genetic changes associated with cancer, studying genetic susceptibility to disease, and measuring response to various types of treatment. Such procedures have also found expanding uses in detecting and quantitating microorganisms in foodstuffs, environmental samples, seed stocks, and other types of material where the presence of specific microorganisms may need to be monitored. Other applications are found in the forensic sciences, anthropology, archaeology, and biology where measurement of the relatedness of nucleic acid sequences has been used to identify criminal suspects, resolve paternity disputes, construct genealogical and phylogenetic trees, and aid in classifying a variety of life forms.
A common method for detecting and quantitating specific nucleic acid sequences is nucleic acid hybridization. This method is based on the ability of two nucleic acid strands which contain complementary or essentially complementary sequences to specifically associate, under appropriate conditions, to form a double-stranded structure. To detect and/or quantitate a specific nucleic acid sequence (known as the “target sequence”), a labelled oligonucleotide (known as a “probe”) is prepared which contains sequences complementary to those of the target sequence. The probe is mixed with a sample suspected of containing the target sequence, and conditions suitable for hybrid formation are created. The probe hybridizes to the target sequence if it is present in the sample. The probe-target hybrids are then separated from the single-stranded probe in one of a variety of ways. The amount of label associated with the hybrids is measured.
The sensitivity of nucleic acid hybridization assays is limited primarily by the specific activity of the probe, the rate and extent of the hybridization reaction, the performance of the method for separating hybridized and unhybridized probe, and the sensitivity with which the label can be detected. Under the best conditions, direct hybridization methods such as that described above can detect about 1×105 to 1×106 target molecules. The most sensitive procedures may lack many of the features required for routine clinical and environmental testing such as speed, convenience, and economy. Furthermore, their sensitivities may not be sufficient for many desired applications. Infectious diseases may be associated with as few as one pathogenic microorganism per 10 ml of blood or other specimen. Forensic investigators may have available only trace amounts of tissue available from a crime scene. Researchers may need to detect and/or quantitate a specific gene sequence that is present as only a tiny fraction of all the sequences present in an organism's genetic material or in the messenger RNA population of a group of cells.
As a result of the interactions among the various components and component steps of this type of assay, there is almost always an inverse relationship between sensitivity and specificity. Thus, steps taken to increase the sensitivity of the assay (such as increasing the specific activity of the probe) may result in a higher percentage of false positive test results. The linkage between sensitivity and specificity has been a significant barrier to improving the sensitivity of hybridization assays. One solution to this problem would be to specifically increase the amount of target sequence present using an amplification procedure. Amplification of a unique portion of the target sequence without requiring amplification of a significant portion of the information encoded in the remaining sequences of the sample could give an increase in sensitivity while at the same time not compromising specificity. For example, a nucleic acid sequence of 25 bases in length has a probability of occurring by chance of 1 in 425 or 1 in 1015 since each of the 25 positions in the sequence may be occupied by one of four different nucleotides.
A method for specifically amplifying nucleic acid sequences termed the “polymerase chain reaction” or “PCR” has been described by Mullis et al. (See U.S. Pat. Nos. 4,683,195, 4,683,202 and 4,800,159 and European patent applications 86302298.4, 86302299.2, and 87300203.4 and Methods in Enzymology, Volume 155, 1987, pp. 335–350). The procedure uses repeated cycles of primer-dependent nucleic acid synthesis occurring simultaneously using each strand of a complementary sequence as a template. The sequence which is amplified is defined by the locations of the primer molecules that initiate synthesis. The primers are complementary to the 3′-terminal portion of the target sequence or its complement and must complex with those sites in order for nucleic acid synthesis to begin. After extension product synthesis, the strands are separated, generally by thermal denaturation, before the next synthesis step. In the PCR procedure, copies of both strands of a complementary sequence are synthesized.
The strand separation step used in PCR to separate the newly synthesized strands at the conclusion of each cycle of the PCR reaction is often thermal denaturation. As a result, either a thermostable enzyme is required or new enzyme must be added between thermal denaturation steps and the initiation of the next cycle of DNA synthesis. The requirement of repeated cycling of reaction temperature between several different and extreme temperatures is a disadvantage of the PCR procedure. In order to make the PCR convenient, expensive programmable thermal cycling instruments are required.
The PCR procedure has been coupled to RNA transcription by incorporating a promoter sequence into one of the primers used in the PCR reaction and then, after amplification by the PCR procedure for several cycles, using the double-stranded DNA as template for the transcription of single-stranded RNA. (See, e.g. Murakawa et al., DNA 7:287–295 (1988).
Other methods for amplification of a specific nucleic acid sequence comprise a series of primer hybridization, extending and denaturing steps to provide an intermediate double stranded DNA molecule containing a promoter sequence through the use of a primer. The double stranded DNA is used to produce multiple RNA copies of the target sequence. The resulting RNA copies can be used as target sequences to produce further copies and multiple cycles can be performed. (See, e.g., Burg, et al., WO 89/1050 and Gingeras, et al., WO 88/10315.)
Methods for chemically synthesizing relatively large amounts of DNA of a specified sequence in vitro are well known to those skilled in the art; production of DNA in this way is now commonplace. However, these procedures are time-consuming and cannot be easily used to synthesize oligonucleotides much greater in length than about 100 bases. Also, the entire base sequence of the DNA to be synthesized must be known. These methods require an expensive instrument capable of synthesizing only a single sequence at one time. Operation of this instrument requires considerable training and expertise. Methods for the chemical synthesis of RNA have been more difficult to develop.
Nucleic acids may be synthesized by techniques which involve cloning or insertion of specific nucleic acid sequences into the genetic material of microorganisms so that the inserted sequences are replicated when the organism replicates. If the sequences are inserted next to and downstream from a suitable promoter sequence, RNA copies of the sequence or protein products encoded by the sequence may be produced. Although cloning allows the production of virtually unlimited amounts of specific nucleic acid sequences, due to the number of manipulations involved it may not be suitable for use in diagnostic, environmental, or forensic testing. Use of cloning techniques requires considerable training and expertise. The cloning of a single sequence may consume several man-months of effort or more.
Relatively large amounts of certain RNAs may be made using a recombinant single-stranded-RNA molecule having a recognition sequence for the binding of an RNA-directed polymerase, preferably Qβ replicase. (See, e.g., U.S. Pat. No. 4,786,600 to Kramer, et al.) A number of steps are required to insert the specific sequence into a DNA copy of the variant molecule, clone it into an expression vector, transcribe it into RNA and then replicate it with Qβ replicase.